Cavities are often filled with a material for insulation or other purposes. In one instance this can for example be a tank with double walls where the cavity between the walls is filled with cement or other hardening material. In another instance it can be a special purpose building, for example a power station having walls where the cavity is filled with cement. Sometimes it may be necessary to ascertain the quality of the filling where there are difficulties doing so due to inaccessibility or safety reasons.
One typical example of such a cavity is the annular space between the casing strings of a hydrocarbon well. A typical hydrocarbon well construction consists of a number of coaxial pipes called casing strings that are successively installed in the well as the drilling progresses. Normally, the first pipe (i.e., the conductor pipe) is set in the well by being bonded to the surrounding formation with cement that is pumped down the pipe and allowed to flow up in the space between the conductor pipe and the surrounding ground. Then, after drilling further down a second casing normally called a surface casing is installed in the well and again the casing is set by filling the annular space between the pipe and the borehole and conductor pipe with cement. Then, depending on the length of the hole drilled and the rock structure, successive casing strings with diminishing diameters are introduced into the borehole and hung off from the wellhead. These casings are normally cemented only partway up from the bottom of the borehole. Lastly, production tubing is installed into the well down to the producing formation and the casings are perforated to allow fluids to enter the well and flow up through the tubing and the Christmas tree and into a flowline.
When cementing each pipe the normal practice is to calculate the amount of cement needed, based on the annular space and the length of the space designed to be filled. However, it is often difficult to calculate the exact amount of cement needed, and the cement level may be lower than intended. In the case of surface casing it is desirable to fill the annular space all the way up to the mudline (i.e., the seabed), but this may not always be achieved, leading to so-called cement shortfall. The top of the surface casing may therefore be filled with a fluid (e.g., water or brine) instead of cement resulting in the surface casing string not being bonded to the conductor pipe all the way up to the mudline. In such a case the part of the surface casing that is not cemented can be regarded as a free-standing column that, if subjected to loads, can be damaged.
The surface casing carries a wellhead and is the principal load-carrying structure for the equipment mounted on top of the wellhead. It serves both the purpose of being a foundation for external loads, such as production equipment (e.g., the Christmas tree) and for borehole support against the formation. A well will be subjected to various loads during its lifetime. In for example a workover situation, a BOP and riser are attached to the Christmas tree, with the riser extending to the surface. The movements of the riser and the use of drilling equipment can set up cyclic loads in the wellhead and the surface casing string (See FIG. 1). This may induce fatigue in the casing string.
Another cause of loads comes from the casing strings being subjected to loads from being heated by the producing fluids.
If the cement has filled the annular space completely and, in addition, has bonded properly with the steel pipe, cyclic loads will be spread along the length of be casing and transferred to the conductor pipe and the ground. However, if there is a length that has not been properly filled, that part of casing can act as a free-standing column (as described above) and cyclic loads can lead to fatigue and damage of the casing. It is also possible that the point at the top of cement level can act as a breaking point because of the movements of the column above.
Similarly, heating and cooling of the casing may induce loads that can lead to fatigue problems and deformation of the casing.
As can be understood from the above, it is therefore of prime interest to find out if the cement job is properly executed, i.e., if the annular space is properly filled. The main purpose of the invention is therefore to find he level of the cement from which the length of the column can be determined.
If later work has to be performed on the well, the BOP and riser are reattached to the Christmas tree so that operations can be carried out in a safe manner.
Both during drilling and (if necessary) workover operations the wellhead is subjected to external loads, as explained above. How this affects the wellhead depends on the length of the free standing column. A longer column will be more vulnerable to fatigue. If the length of the free standing column can be determined, how much load the wellhead can be subjected to can be calculated and this will in turn determine how much work can be done. This enables an operator to predict the operational lifetime of the well and to ensure the integrity of the well structure.
One method for non-destructive logging of layers of different materials comprises the creation of a magnetic pulse within a pipe to cause the pipe to act as an acoustic transmitter. One such example is disclosed in U.S. Pat. No. 6,595,285, which describes a method and device for emitting radial seismic waves using electromagnetic induction to generate a magnetic pressure pulse that causes a distortion within a pipe and which utilizes the elastic restoring property of the pipe to cause it to become an acoustic transmitting device. This can be used for generating seismic waves in the subsoil. In U.S. Pat. No. 3,752,257 a similar device is located within a conductor pipe and used to measure acoustic velocity within a formation. The acoustic signals are reflected from the formation and recorded by two receivers and the delta travel time between the receivers is recorded. It is also stated that this apparatus can be used to measure the quality of the cement bond between the conductor pipe and the earth formation. However, there is no further explanation on how this may be achieved, and the inventors have found that this is not a reliable way of determining the cement level.
In both these examples of the known art the transmitter is located such that the acoustic waves only have to traverse one pipe wall, e.g., the conductor pipe. If the device is to be located in a fully completed well there is the challenge of creating a signal that is both strong enough to penetrate through several different casing pipes and be able to distinguish between the reflected signals from the various casings.
In WO 2011/117355 belonging to the applicant, this problem is addressed by using a signal of very short duration. Because of the short duration of the signal it is possible to separate the reflections on a time lapse basis. The speed of the acoustic waves is different in cement (i.e., a solid) than in water. When transmitting signals at various points in the well it will be possible to find the spot where the signal is different. This, in theory, marks the exact location of the top of the level of cement.
In addition to the problem of separating the various reflections from each other, there is also the problem with signal noise. This can be signal noise generated by the system itself, but also second and third reflections from the various casings. The latter of course becomes even more complicated when the reflected signal comes from an annulus that is several layers away from the receiver, as is the case of the annulus between the conductor and the surface casing, which is known in the art as the “D” annulus. Both the transmitted and the reflected signal must in this case pass through four casing pipes. There may also be reflected signals travelling along the pipe that also can produce noise.
In view of the above background, there is a need for an improved method for determining a position of a water/cement boundary in an annular area between two concentric pipes in a hydrocarbon well.